FIG. 7 is a block diagram illustrating a conventional example of an AD conversion circuit. In such a conventional circuit, an AD conversion circuit X for converting an analog signal SA into a digital signal SD may be required in a front stage of a digital signal processor (DSP) 301 so that the analog signal SA is processed by the DSP 301.
The AD conversion circuit X according to the conventional example includes an AD converter (ADC) 302 with sampling frequency fs, a programmable gain amplifier (PGA) 303, and an analog filer (AFLT) 304. In the AD conversion circuit X, the analog filter 304 (also referred to as an antialiasing filter) is disposed in a front stage of the ADC 302 in order to prevent generation of aliasing (or folding noise) therefrom.
FIG. 8 is a view illustrating analog filter characteristics (or frequency-gain characteristics) according to a conventional example. As illustrated in FIG. 8, the analog filter 304 is a type of a low pass filter for removing a frequency component higher than a maximum frequency fh of an analog signal band.
In order to prevent generation of aliasing in the entire analog signal band, a condition of fh<fs/2l (=Nyquist frequency fn) needs to be met according to sampling theorem. Thus, the conventional AD conversion circuit X is configured such that an unnecessary high frequency component (>fh) is removed as much as possible from the analog signal SA by using the higher order (e.g., ninth order) analog filter 304 and the ADC 302 (e.g., fs=62.5 Msps with respect to fh=30 MHz) having a sampling frequency fs twice or greater than a maximum frequency fh of an analog signal band is used.
However, in the conventional AD conversion circuit X, the higher order analog filter 304 may be provided as an external component (passive element such as an inductor L, or a capacitor C) in order to reduce a circuit area and the power consumption, or the like of a semiconductor device 300. Thus, the conventional AD conversion circuit X may be problematic in that the number of external components and manufacturing cost increase.